210 IMMUNO-CATALYSIS 



antitoxin molecule for two of toxin (Pappenheimer, 1940; Pappen- 

 heimer, Lundgren and Williams, 1940). Boyd (1947) has tabulated 

 molecular compositions of various antigen-antibody complexes. In the 

 presence of extreme excess of antibody, serum albumin can combine 

 with six rabbit antibody molecules; one molecule of thyroglobulin 

 (mol. wt. 650,000) combines with forty rabbit antibody molecules; 

 ovalbumin with four (horse) or five rabbit antibody molecules; and 

 nine hundred rabbit antibody molecules with one of tobacco mosaic 

 virus. The high molecular composition of certain antibodies with the 

 respective antigens is understandable if we take into consideration 

 the molecular weights of these antigens. The molecular weight of 

 tobacco mosaic virus is 60 million. The molecular weights of diph- 

 theria toxin and serum albumin are about 70,000, and that of ovalbu- 

 min about 40,000. 



Since the size of rabbit antibody globulin to various antigens is 

 constant, the critical factor in the molecular composition of antigen 

 and antibody complexes is the molecular size and the surface areas of 

 antigens. The ratios of the molecular weight of the tobacco mosaic 

 virus to that of the serum albumin or diphtheria toxin is about 860. 

 One virus particle combining with 900 rabbit antibody molecules 

 would be equivalent to a combination of one molecule of antibody and 

 one virus unit of 70,000 molecular weight. This approximation would 

 be valid if the shape of the virus and that of the serum albumin or 

 diphtheria toxin are of comparable dimensions. The ratio of the major 

 to minor axis of the diphtheria toxin is 4.7, and that of the virus 

 is 18. Weight for weight, virus particles may therefore possess a three 

 times greater surface area. Using this rough relationship, one can see 

 that only a very few antibody molecules can combine with one sub- 

 molecular unit of virus antigen. Under these conditions, the formation 

 around the antigen molecule of a barrier impermeable to a substrate 

 does not appear to be probable. 



Let us examine other data which would seem to throw some light 

 on this question. It had been assumed that when antibodies combine 

 with microorganisms, causing clumping or agglutination, they simply 

 reduce the effective surface relationship between the enzyme and sub- 

 strate (Taliaferro, 1948). Under these conditions it had been assumed 

 that substrates are prevented from reaching the enzyme sites. Sevag 

 and Miller (1948) found that agglutinated pneumococci with or with- 



